Increasing price of energy sources due to depletion of fossil fuels and an increased interest in environmental pollution have brought about increased demand for environmentally friendly alternative energy sources as an indispensable element for future life. Studies on various power generation technologies such as nuclear, solar, wind, and tidal power generation technologies have continued to be conducted and power storage devices for more efficient use of such generated energy also continue to be of great interest. Secondary batteries have been used as such power storage devices. Among secondary batteries, lithium secondary batteries have begun to be used for mobile devices and, along with increasing demand for reduced weight and high voltage and capacity, recently, use of lithium secondary batteries has been significantly extended to electric vehicles, hybrid electric vehicles, and auxiliary power sources based upon smart grid.
However, numerous challenges, which have yet to be addressed, remain before lithium secondary batteries can be used as large-capacity power sources. One important challenge is to improve energy density and increase safety. Another important challenge is to reduce process time and to achieve uniform wetting for large-area electrodes. Many researchers have conducted intensive studies on materials that can satisfy low cost requirements while increasing energy density and have also put effort into studies on materials for improving safety.
Ni-based or Mn-based materials having higher capacity than LiCoO2, which has been conventionally used, are typical examples of materials being studied for energy density improvement. Materials that are based on Li alloying reactions with Si or Sn rather than based on intercalation reactions are typical examples of materials for anodes being studied as alternatives to conventional graphite-based materials.
A stable olivine-based cathode active material such as LiFePO4, a cathode active material such as Li4Ti5O12, or the like have been studied to improve safety. However, such materials for safety improvement inherently have a low energy density and do not fundamentally solve safety problems associated with the structure of lithium secondary batteries.
Secondary battery safety may be largely divided into internal safety and external safety and may further be divided into electrical safety, impact safety, and thermal safety. Occurrence of these safety problems commonly entails temperature increase, which necessarily results in contraction of a stretched separator that is generally used.
Although many researchers have suggested all-solid-state batteries to resolve this safety issue, all-solid-state batteries have a lot of problems in replacing batteries available on the market.
First, currently used electrode active materials are in a solid state. Thus, when a solid electrolyte or a polymer electrolyte is used, the surface area of the electrolyte in contact with the active material for lithium ion movement is significantly reduced. Therefore, the ionic conductivity is very low even when the solid or polymer electrolyte has a conductivity of 10−5 s/cm, similar to the current liquid electrolyte. Second, the ionic conductivity at the solid-solid interface or the solid-polymer interface should be much lower for the same reason. Third, even when a solid electrolyte with high conductivity is used, the ionic conductivity is still very low due to a polymer binder that should be employed to provide binding force that is essential to battery formation. Fourth, to form a battery, not only the separation layer needs to have ionic conductivity but the cathode and anode active materials also need to employ materials for ionic conductivity improvement to increase ionic conductivity of the electrodes. However, if a solid electrolyte or a polymer electrolyte is included as an electrode component, the capacity is reduced.
Thus, there is a great need to provide a battery structure that prevents short-circuiting due to separator contraction and provides excellent battery performance.